They sound like Locusts. I prefer Locusts to crickets.
I would recommend size small/medium for him.

Are you supplementing all is food with calcium and nutrobal? Only, it could be the photo, but he looks like he could have a kink in his tail, and also an over shot lower jaw, which could be a sign of MBD.... but seen as you haven't had him long, his last owners would have not given him enough calcium and UV light.

Have you had him at the vets for a check over?